1. Field of the Invention
This invention relates to a condensed polycyclic, polynuclear aromatic thermosetting resin having good moldability and heat resistance which are particularly improved with respect to the formation of a cloud on a mold during molding, thereby making it possible to produce molded articles with significantly improved surface gloss. This invention also concerns a method for the preparation of the aromatic thermosetting resin and a molding compound comprising the resin.
2. Description of the Prior Art
Thermosetting resins such as phenolic resins, urea-formaldehyde resins, melamine-formaldehyde resins, epoxy resins, unsaturated polyester resins, and polyimide resins find a wide variety of applications on account of having heat resistance which is superior to that of thermoplastic resins. However, except for polyimides, the long-term service temperature of these resins is on the order of 150.degree. C. to at most 200.degree. C.
Polyimide resins have extremely good heat resistance and their long-term service temperature is between 200.degree. C. and 300.degree. C. However, they are expensive and have a drawback of relatively poor moldability.
Recently one of the inventors found a novel class of thermosetting resins which are condensed polycyclic, polynuclear aromatic hydrocarbon resins having good heat resistance. These novel resins are prepared by reacting a condensed polycyclic aromatic compound or a mixture comprising it in a major amount with a crosslinking agent which is an aromatic compound having at least two hydroxymethyl or halomethyl groups at an elevated temperature in the presence of an acid catalyst. These resins, which are called COPNA resins for short are disclosed in Japanese Patent Applications Kokai Nos. 62-521 and 62-522 (1987).
In general, aromatic hydrocarbon resins including the above-mentioned condensed polycyclic polynuclear aromatic hydrocarbon resins are advantageous in that they have good heat resistance and can be easily modified because they can be readily subjected to co-condensation with aromatic compounds having a variety of functional groups. Thus, it is possible to endow these resins with a wide variety of properties or with a special function. For this reason, the COPNA resins are considered to hold great promise.
In addition to good heat resistance, thermoset moldings of the aromatic hydrocarbon resins (COPNA resins) have good electrical insulating properties and good resistance to humidity and chemicals. Therefore, they are likely to find applications in many fields for single uses or for use as a matrix resin to form a composite.
However, the COPNA resins have the drawback that when they are shaped and cured in a mold, a considerable amount of pale green deposits is formed on the inner surface of the mold. The formation of such deposits is called "clouding" since the deposits resemble a cloud, and it significantly deteriorates the surface gloss of the resulting molding. If a clouded mold is used repeatedly in continuous molding, mold release gradually becomes difficult. In some cases, molding may become impossible with the mold after it has been used several times. Therefore, it is necessary or desirable to clean the mold frequently or in some cases after each cycle of molding, but this cleaning is burdensome and greatly decreases the productivity of moldings. Thus, although the COPNA resins have good heat resistance and other desirable physical, electrical, and chemical properties, they suffer from the serious problem of poor moldability caused by cloud-formation on the mold.